Disturbances following a student march to demand free, quality public education left 20 police injured and 250 protesters in custody, Chilean authorities said.

"We had a day of violence, worrisome. We suffered an attack on one of our vans, we were witnesses to an attack on a truck carrying prison guards, on a transit van and to various incidents of vandalism," police Gen. Luis Valdes told the press.

Hooded assailants in Santiago sacked supermarkets, pharmacies and a cell phone store, and destroyed 15 traffic lights and 50 street signs.

The governor of the capital metropolitan region, Cecilia Perez, demanded that the student organizations behind the march take it open themselves to condemn the violence.

"This brutality carried out by a group of criminals must be rejected by all social actors without hesitation, but even more forcefully by those who today called this protest," she said.

Organizers said 150,000 people took part in Thursday's march, though police put the number at 40,000. Even if the smaller figure is correct, the demonstration signaled the continuing strength of a student movement that took the streets more than 40 times in 2011.

Students are unhappy with a highly stratified education system that funnels state subsidies to private institutions even as public schools in poor areas struggle.

Chile's public schools and universities were neglected by the 1973-1990 dictatorship of the late Gen. Augusto Pinochet, who embraced doctrinaire free market policies.

Private schools mushroomed under the military regime and the trend continued after democracy was restored, even during the 1990-2010 tenure of the center-left Concertacion coalition.

President Sebastian Piñera, a right-wing billionaire who thrived during the Pinochet era, has taken some steps to make college more affordable for low-income students.

Thursday's march followed the release last week of a congressional report accusing seven private universities of violating a legal requirement that educational institutions operate on a non-profit basis.

Corporations running private colleges often skirt the law by establishing parallel real estate companies that own campus land and buildings and lease them to the educational institutions at a profit. EFE